The invention is embodied in an apparatus and method for modulating the strength of a grating such as a long period grating (LPG) within optical systems and devices by varying the light transmission and loss characteristics of the cladding mode, rather than varying the effective refractive index of the fiber layers. Varying the transmission characteristics of the cladding mode correspondingly affects the strength of the grating (i.e., amplitude modulation).
According to embodiments of the invention, the use of a light-scattering or light-absorptive material in the cladding of the optical fiber or other optical energy transmission medium causes the cladding to vary or switch between a first state that effectively allows coherent coupling to the cladding modes and a second state that effectively prevents coherent coupling to the cladding modes. The light-scattering materials include electro-optic materials that cause the cladding to switch between the first and second states based on the presence (or absence) of an electric field, magneto-optic materials that cause the cladding to switch between the first and second states based on the presence (or absence) of a magnetic field, and materials capable of phase transitions that cause the cladding to switch between the first and second states based on temperature. The light-absorptive materials include dopants that cause the cladding to switch between the first and second states based on the wavelength of the optical energy.
Embodiments of the invention include arrangements in which the cladding includes one or more holes filled with a light scattering or absorptive medium. The holes are formed, e.g., longitudinally down the length of the cladding and generally parallel to the fiber core. Alternative embodiments of the invention include one or more light-scattering or light-absorptive coatings applied to the outer surface of the cladding, and one or more light-scattering or light-absorptive regions within the cladding that are coaxial with the fiber core.
Embodiments in accordance with the invention differ from conventional optical media in that the cladding mode loss (attenuation) is varied rather than varying the index of refraction in conventional arrangements. The magnitude of the loss according to embodiments of the invention depends on the specification arrangements employed, but such loss typically is measured in decibels (dB) per unit length in the given cladding mode.